Shape memory alloy and method of treating the same

ABSTRACT

A method of treating a shape memory alloy to improve its various characteristics and to cause it to exhibit a two-way shape memory effect. A raw shape memory alloy having a substantially uniformly fine-grained crystal structure is prepared and then its crystal orientations are arranged substantially in a direction suitable for an expected operational direction, such as tensile or twisting direction or the like, in which the shape memory alloy is expected to move when used in an actuator after the completion of the treatment.

This application is a divisional of application Ser. No. 09/871,619,filed on Jun. 4, 2001, now U.S. Pat. No. 6,596,102, the entire contentsof which are hereby incorporated by reference and for which priority isclaimed under 35 U.S.C. § 120; and this application claims priority ofapplication Ser. No. 2000-204927 filed in Japan on Jul. 6, 2000 under 35U.S.C. § 119.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a shape memory alloy (SMA) suitable foractuators and a method of treating the same.

2. Related Art

Heretofore, upon treating a raw shape memory alloy so as to make itsuitable for use in actuators, generally it has not been done to refinecrystal grains and control crystal orientations of the raw shape memoryalloy.

On the other hand, in order to use a shape memory alloy, it is necessaryto impart a required shape to the shape memory alloy, and therefore toperform a heat treatment peculiar to each kind of shape memory alloy.This heat treatment is called shape memory treatment and it is necessaryto strictly control various conditions thereof, as it is a very delicatetreatment. For example, the following methods have been well known asshape memory treatments for common Ti—Ni based shape memory alloys. Thefirst method, which is referred as medium temperature treatment, is theone wherein a shape memory alloy is sufficiently work hardened and thencold worked into a desired shape, and thereafter, held at a temperatureof 400 to 500° C. for a few minutes to several hours with the desiredshape being restrained. The second method, which is referred as lowtemperature treatment, is the one wherein a shape memory alloy is heldat a temperature of 800° C. or above for some time, thereafter rapidlycooled and cold worked into a desired shape, and then held at a lowtemperature of 200 to 300° C. with the desired shape being restrained(Illustrated idea collection of applications of shape memory alloys inthe latest patents, written and edited by Shoji Ishikawa, Sadao Kinashiand Manabu Miwa, published by Kogyo-chousa-kai, pp. 30).

In general, conventional shape memory alloys suffer from the followingshortcomings when used in actuators.

(a) The response characteristic (speed) is inferior.

(b) Usable temperature range is restricted, since M_(s) and M_(f) points(M_(s) being the temperature at which the martensite phasetransformation starts and M_(f) being the temperature at which themartensite phase transformation ends) are difficult to be raised.

(c) Only a small force can be effectively extracted from the shapememory alloy.

(d) The service life before being broken is short.

(e) The shape memory alloy tends to lose the memory of an impartedconfiguration and permanent strain tends to be produced in the shapememory alloy for a short period of time.

(f) The strain which can be extracted from the shape memory alloy as amovement (hereinafter referred as operational strain) is decreased for ashort period of time.

(g) Shape memory alloy materials, such as Ti—Ni based or Ti—Ni—Cu basedalloys and the like, which are intermetallic compounds having strongcovalent bonding characteristic and are difficult to work, are difficultto use when they are in certain compositions, since they are verybrittle and fragile.

With such shortcomings, 80 to 90% or more of applications of shapememory alloys have been those wherein they are used as superelasticspring materials and only the rest has been directed to actuators.Moreover, most of the shape memory alloys for use in actuators have beenformed into the shape of a coil spring, wire or plate and have beenexpected to be reverted from a configuration deformed by bending ortwisting and bending to the original configuration upon application ofheat (in case the shape memory alloy is formed into a coil spring shape,though macroscopically or apparently it is deformed as if it wereelongated or compressed upon application of a force thereto, in a truesense the deformation it is subject to is a twisting and bending one).The reason for utilizing reversion from a bending deformation ortwisting and bending deformation as stated above has been that the shapememory alloy should be used so that its small strains may be multipliedsince the range of its shape memory effect (SME) stably available isvery narrow. Though it is said that, in conventional shape memoryalloys, the maximum operational strain reaches a few percent to about 10percent, this is true only when deformation and shape recovery areperformed only once or a few times. Practically speaking, whendeformation and shape recovery are repeated over large cycle numberswith regard to the conventional shape memory alloy, the operationalstrain is decreased and the alloy loses the memory of the impartedconfiguration and eventually is broken.

All of the conventional shape memory treatments intend to keep the shapestability while obtaining the pseudoelasticity and shape memory effectby partly producing microstructures which can cause pseudoelasticity andshape memory effect in microstructures of the shape memory alloystrengthened by work hardening. In other words all of the conventionalshape memory treatments are those which obliges to sacrificepseudoelasticity and shape memory effect to some extent to keep shapestability.

On the other hand, the present inventor has disclosed in U.S. Pat. No.4,919,177 a method of treating Ti—Ni based shape memory alloy wherein aTi—Ni based polycrystalline shape memory alloy material is subjected toa heat cycle which rises and drops over the transformation region of theshape memory alloy as well as to a directional energy field. Accordingto this method, the crystal orientations of the shape memory alloy arerearranged along a specific direction and the disadvantages of theconventional shape memory alloy are overcome considerably.

However, in the method disclosed by the present inventor, the crystalgrains of the shape memory alloy are not refined but caused to grow insize. Besides, since a tensile force is applied to the shape memoryalloy in the final step of arranging the crystal orientations, there isa tendency that the microstructure of the shape memory alloy finallyobtained is destroyed by the tensile force. Therefore, it is still notenough in overcoming the disadvantages of the conventional shape memoryalloy.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide a shapememory alloy having a good response characteristic and a method oftreating a shape memory alloy for obtaining such a shape memory alloy.

It is another object of the present invention to provide a shape memoryalloy which can be used over a wide range of temperature and a method oftreating a shape memory alloy for obtaining such a shape memory alloy.

It is still another object of the present invention to provide a shapememory alloy from which a greater force can be practically andeffectively extracted and a method of treating a shape memory alloy forobtaining such a shape memory alloy.

It is a further object of the present invention to provide a shapememory alloy from which great operational strains can be extracted overlarge cycle numbers and a method of treating a shape memory alloy forobtaining such a shape memory alloy.

It is a still further object of the present invention to provide a shapememory alloy exhibiting a huge two-way shape memory effect (reversibleshape memory effect) and a method of treating a shape memory alloy forobtaining such a shape memory alloy.

It is another object of the present invention to provide a shape memoryalloy having a long service life and a method of treating a shape memoryalloy for obtaining such a shape memory alloy.

It is still another object of the present invention to provide a shapememory alloy which does not lose its memorized shape easily and a methodof treating a shape memory alloy for obtaining such a shape memoryalloy.

It is a further object of the present invention to provide a shapememory alloy of which operational strain diminishes less even with anincrease of a deformation-recovery cycle number and a method of treatinga shape memory alloy for obtaining such a shape memory alloy.

It is a still further object of the present invention to provide a shapememory alloy which exhibits stably the aforesaid various excellentproperties over large cycle numbers for a long period of time and amethod of treating a shape memory alloy for obtaining such a shapememory alloy.

It is another object of the present invention to provide a method oftreating a shape memory alloy which makes it possible to employ, as rawmaterials, those shape memory materials which have been regarded asdifficult to use because of their brittleness and easiness to crack andconvert them into ductile shape memory alloys in the shape of a wire orsheet etc.

It is yet another object of the present invention to provide a method oftreating a shape memory alloy which makes it possible to arrange crystalorientations of a shape memory alloy without damaging the microstructureof the alloy.

Crystal grains of a shape memory alloy have orientations and there exista plurality of orientations along which reversible slips or shearingdeformations (variants), wherein microscopically relative moving rangesbetween the atoms of the alloy are restricted, can appear, though theyare limited in number. For example, in case of a Ti—Ni based shapememory alloy, there are as much as twenty four (24) orientations alongwhich the deformations referred to as variants can occur. In the presentinvention, the crystal orientations of the shape memory alloy arearranged substantially along a direction suitable for an expectedoperational direction, in other words, a direction suitable for amovement of the shape memory alloy in the expected operationaldirection. The term “expected operational direction” as herein usedmeans a direction such as a tensile or twisting direction or the like inwhich the shape memory alloy is expected to move when used in anactuator after the completion of the treatment. For example, when ashape memory alloy in the wire shape is used in a contraction-relaxationfashion, the expected operational direction is a tensile direction,while when a shape memory alloy in the coil spring shape is used, theexpected operational direction is a torsion direction. (In case a shapememory alloy in the coil spring shape is used, it performs shaperecovery from a twisting and bending deformation upon heating.Therefore, strictly speaking, it may be said that the expectedoperational direction in this case is a torsion and bending direction.However, the substantial expected operational direction is a torsiondirection, because bending deformation comprises a negligiblepercentage.)

A method of treating a shape memory alloy in accordance with the presentinvention comprising the steps of providing a raw shape memory alloywith a substantially uniformly fine-grained crystal structure; andarranging crystal orientations of the raw shape memory alloysubstantially along a direction suitable for an expected operationaldirection.

It is preferred that the average grain size of the raw shape memoryalloy is selected to be 10 microns or less in the step of providing theraw shape memory alloy with a substantially uniformly fine-grainedcrystal structure. Most preferred is the average grain size in the rangeof 1 micron to several microns or less. With such grain size, the shapememory alloy after the completion of the treatment is particularlystable when subjected to deformation-recovery cycle.

In general, specific characteristic properties of crystalline materialsare based on the phenomena in crystal grains of the materials.Accordingly, in many cases, these specific characteristic propertiesshould naturally be most remarkably recognized when the materials are ofsingle crystal. For this reason, when the excellent properties orfunctions of some material are to be utilized, in general, the bestresults can be obtained when the material is of single crystal.Basically the shape memory alloy is no exception on this matter. A shapememory alloy of single crystal can be deformed in a slip direction byvery small force in the range where reversible slip deformation canoccur under a low temperature at which it is in the martensitic phase asa whole (Slip deformation in this specification means shearingdeformation which is the cause of the shape memory effect and whereinreversible movement is possible within a limited range, but it does notmean permanent and continuous slip between atoms which is the cause ofthe plastic deformation).

However, in practice it is extremely difficult to industrially producethe single crystalline material, and the production of it, even whenachieved, should be very expensive. Besides, in case of a shape memoryalloy, when it is of single crystal, its microstructure becomesunstable.

Of course conventional shape memory alloys are polycrystallinesubstances, and in general, orientations of the respective crystalsthereof have been random and the grain sizes of respective crystals areuneven, and thereby it is thought that aforesaid various shortcomingsare caused (this will be discussed later in detail).

The present inventor has found that a shape memory alloy can be obtainedwhich has both advantages of the single crystalline shape memory alloyand those of the conventional polycrystal shape memory alloys, when theshape memory alloy, as in the present invention, is formed of apolycrystal material and provided with a substantially uniformlyfine-grained crystal structure, and the crystal orientations thereof arearranged along a direction suitable for an expected operationaldirection. When the crystal grain sizes of the alloy are madesubstantially uniform and the crystal orientations are arranged along adirection suited for a desired movement of the alloy, even if giganticshape recovery force is produced in respective crystal grains, no partof the alloy is subject to an excessive deformation and the internalstructure of the alloy is difficult to destroy. Besides, when respectivecrystal grains are adequately small, structural contradictions caused bydifferences between deformation directions of the respective crystalgrains, etc. are also small and thereby the respective crystal grainsthemselves are difficult to destroy. Moreover, in such a material, sincethe volume proportion of the structure at and around the crystal grainboundaries to that within the grains is comparatively larger, itsability to absorb the structural contradictions is high. Further, such amaterial can be reformed into a shape memory alloy in the shape of awire or sheet etc. which is sufficiently ductile over a wide strainrange, even in the case where it is brittle when it is a raw material.The reason for this is presumed that, in such a material, the structureat and around the crystal grain boundaries exhibits properties likethose of an amorphous material. Even the respective crystal grains arefine, if the crystal orientations are arranged, comparatively largeshape memory effect can be extracted from the shape memory alloy. Aforce required to deform the shape alloy is small, since theorientations of the respective crystals along which the crystals areeasy to move are arranged in the same direction. Because the volumeproportion of the structure at and around the crystal grain boundariesto that within the grains is comparatively larger, large elastic energycan be stored at and around the crystal grain boundaries withoutemploying the measures of depositing impurities there, or the like, andthereby a stable and large two-way shape memory effect can be obtainedas well as the property that a force required to deform the alloy issmall.

Thus, the shape memory alloy in accordance with the present inventionhas the following excellent properties, though some of them have beenalready mentioned above.

(A) Since the temperature hysteresis is small on the temperature-stressdiagram and the transformation temperature range is narrow, heating andcooling of the alloy can be taken place quickly, the response of thealloy is good, and a high-speed reciprocating motion can be achieved.For example, when applied to a Ti—Ni—Cu based shape memory alloy, thetemperature hysteresis can be almost zero over a comparatively widerange. A successive reciprocating operational strain reaching to almost80% of that in the full stroke (strain ε=4%) could be successfullyextracted from a shape memory alloy in accordance with the presentinvention with a 150 Mpa load and a temperature difference of merely 10°C. This, when compared to a engine, is equivalent to the revolutionspeed is remarkably raised with the same size. Accordingly, it isequivalent to that the horsepower as well as the load capacity isconsiderably raised. A significant improvement of the responsiveness canbe expected, when used in a mechanism such as a servo actuator whereintwo-way movement is required.

(B) The force which can be practically extracted from the shape memoryalloy (hereinafter referred as recovery force) can be increased. Therecovery force does not depend on the maximum recovery stress but thelimit of the stress repeatedly usable in consideration of fatigue of thealloy, etc. When compared to a engine or motor, the recovery forcecorresponds to the maximum torque. With the shape memory alloy treatedby the method in accordance with the present invention, the limit of thestress practically available in the reiterative operation is high, evenwhen the maximum recovery stress is same as that of the conventionalshape memory alloy. The conventional shape memory alloy has a smallrecovery force, and if operated repeatedly with an excessively largestress applied thereto, it suffers from loss of the memory of theimparted configuration, decrease of the operational strain and rupture,as stated above. It means shortening of service life of the actuator.This is the reason why most of conventional shape memory alloy actuatorshave been formed in the shape of a coil spring, as previously stated.With the coil spring shape, the strain produced in the alloy is verysmall when the alloy is deformed. Therefore, the stress actually usedhas been considerably small as compared with the maximum stresspractically available.

(C) Large operational strains can be extracted over large cycle numbers.The shape memory alloy in accordance with the present invention, whenformed into a rectilinear shape, can achieve a deformation-shaperecovery cycle with a tensile strain of 5% or more. The value of theoperational strain, 5% or more stands comparison with that a 1 m longround bar is expanded and contracted by 5 cm or more. This magnitude ofstrain is much larger than that of the strain which an ordinary coilspring is subject to when it is deformed and restored between the coiland rectilinear shape. This value is much larger than the ranges ofstrains available in case of conventional shape memory alloys includingsuperelastic alloys. When the treatment in accordance with the presentinvention is applied to a brittle raw shape memory alloy material suchas Ti—Ni—Cu based shape memory alloy and the like, huge operationalstrains as stated above can be extracted stably over more than onehundred million cycles. When the conventional shape memory alloy is usedin the coil spring shape, in most cases, the moving strain is less than0.1% in tensile strain equivalent. In other words, in most cases, thecoil spring of a shape memory alloy has been used with almost the samemagnitude of displacement as the coil spring of a non-shape memory alloymetal such as iron and the like.

(D) It is possible to cause a shape memory alloy to exhibit a hugetwo-way shape memory alloy effect. The two-way shape memory effect is aphenomenon wherein a shape memory alloy recovers the originalconfiguration upon heating and deforms into another configuration uponcooling, and no force or only a very small force is required when thealloy is subject to the deformation at a low temperature in a directionopposite to the shape recovery. Apparently, it appears that the shapememory alloy remembers two configurations, the deformed configuration ata low temperature and the original configuration at a high temperature.For instance, in case the shape memory alloy is rectilinear and thedeformed configuration (length) thereof is the one stretched from theoriginal configuration (length), the shape memory alloy contracts to theoriginal length and becomes hard upon heating, while it extends byitself to the deformed length and becomes soft just like a musclerelaxes upon cooling, even in the absence of a load. In other words, theshape memory alloy expands and contracts, driven by heating and coolingalone in the absence of a bias force from the outside. According toliterature, etc., it has been thought that, generally the two-way shapememory effect is a phenomenon observed only within the range wherein astrain ε is 1% or less in tensile strain equivalent and it is difficultto put it to practical use since it is unstable. In fact, hithertodevices utilizing the two-way shape memory effect have been hardlyfound.

According to the present invention, however, it is possible to cause ahuge two-way shape memory effect almost over the whole range wherein theshape memory effect occurs, namely, the whole range of recoverablestrain. According to the present invention, the two-way shape memoryeffect with a strain of 5% can be exhibited even in the absence of aload. The present inventor postulates that, since the polycrystal shapememory alloy in accordance with the present invention has crystals eachof which orientation, size and position are adapted to deformations fromthe outside, a stable two-way shape memory effect can be induced almostin the whole range of the operational strain, if there exists in thealloy the slightest level of a residual stress field resulted from theworking in a direction opposite to the shape recovery direction. Thishuge two-way shape memory effect appears stably over about one hundredmillion cycles in the absence of a load.

(E) The shape memory alloy in accordance with the present invention hasa long service life. The conventional shape memory alloy has a servicelife of about one hundred thousand cycles, at the largest, even with thesmall operational strain. Particularly, in case a movement wherein theoperational strain exceeds 2% in tensile strain equivalent is performed,there is a tendency that its service life becomes extremely short.However, the shape memory alloy in accordance with the present inventionprovides a stable movement over one hundred million cycles with a hugeoperational strain reaching nearly 5%.

(F) The memory of the imparted configuration and the range of theoperational strain are stable, that is, the memory of the impartedconfiguration and the range of the operational strain do not diminishwith cycle number of the deformation and recovery or do only slightly.In other words, the magnitude of the operational strain has littleeffect on the service life of the shape memory alloy. The reason for itis postulated that the shape memory alloy in accordance with the presentinvention has the orientations, sizes and arrangements of the respectivecrystals in a state adapted to deformations from the outside. It ispresumed that the deformation from the outside is undertaken, to acertain extent, mainly by the crystals which achieve a huge reversiblethermo-elastic deformation that is characteristic of a shape memoryalloy, while the deformation larger than it is undertaken by thestructure at and around the crystal grain boundaries wherein areversible thermo-elastic deformation is hardly produced. With suchstructure of the shape memory alloy, displacements, plastic deformationsand rotations of the respective crystal grains are hard to occur evenwith large cycle numbers, and the alloy is hardly subject to a permanentdeformation.

(G) Even when the raw material is brittle, it can be reformed into aductile shape memory alloy in the shape of a wire, sheet or the like.The shape memory alloy in accordance with the present invention hashigher apparent ductility than shape memory alloys treated by theconventional shape memory treatment since it consists of the finecrystal grains reversibly deformable and the structure at and around thecrystal grain boundaries which exhibits amorphous like properties andoccupies a considerable part of the alloy with regard to volume.

(H) The various excellent properties of the shape memory alloy mentionedabove are stable over a long time of period and large cycle numbers.

In a particular aspect of the method of treating a shape memory alloy inaccordance with the present invention, the step of providing a raw shapememory alloy having a substantially uniformly fine-grained crystalstructure comprises the steps of heating the raw shape memory alloy inan amorphous state or a state similar thereto to the temperature atwhich recrystallization begins or a little above for a short period oftime, with a stress applied to the raw shape memory alloy in theexpected operational direction at least in the stage where a recoveryrecrystallization begins, to produce a substantially uniformfine-grained crystal structure with an anisotropy in the expectedoperational direction, while relaxing the internal stress generated inthe raw shape memory alloy in the expected operational direction.

In case the raw shape memory alloy is not in an amorphous state or astate similar thereto, the raw shape memory alloy can be be put into astate similar to amorphous state, for instance, by being subject to asevere cold working. It is preferable that the severe cold working isachieved at a cryogenic temperature which is sufficiently lower than thetemperature singular point B of the raw shape memory alloy. The point Bis an inflection point observed in the sub-zero temperature range and isassociated with transitions of the physical property values such asspecific heat, electrical resistance and the like (This will beexplained later in more detail). The object for this is to completelytransform non-martensite structures remaining in the alloy, even if theamount of them are very small, into the martensite. In general, the socalled martensite finished point M_(f) at which the shape memory alloytransforms completely from austenite to martensite is the temperaturewhich is measured with respect to a specimen completely annealed. Inworked materials, however, there remain a considerable amount of thenon-martensite structures even at this temperature. The non-martensitestructures may be retained austenite, a structure resulted from workhardening or the like.

Upon heating the raw shape memory alloy to the temperature at whichrecrystallization begins or a little above for a short period of time,the raw shape memory alloy may be either in a state where a stress isapplied to it in the expected operational direction or where it isconstrained in a shape not loosened in the absence of a load. At thisstage, since the raw shape memory alloy has a martensitic componentwhich can recover the shape in the expected operational direction uponheating, if it is constrained in a shape not loosened in the absence ofa load, a stress is produced in the expected operational direction whileheating and thereby the same result is obtained as when the alloy isconstrained with a stress applied thereto prior to heating as statedabove. What is essential is that at least when a recoveryrecrystallization begins the raw shape memory alloy is in a state wherea stress is loaded thereto in the expected operational direction.

In the particular aspect of the method of treating a shape memory alloyin accordance with the present invention, the step of arranging crystalorientations of the raw shape memory alloy comprises the steps ofsubjecting the raw shape memory alloy to a high level of deformation bymeans of a stress in the expected operational direction at a very lowtemperature at which the austenite phase does not remain in the rawshape memory alloy so that a slide deformation is introduced into thecrystal grains of the raw shape memory alloy which have been transformedcompletely into the martensite phase, within a reversible range alongthe direction of the stress; heating the raw shape memory alloy to atemperature between A_(f) (a temperature at which the austenitictransformation ends) and the recrystallization temperature with a stressapplied to said raw shape memory alloy in the expected operationaldirection so that the directions of reversible slip motions of therespective crystal grains of said raw shape memory alloy are arranged inthe expected operational direction.

The crystal orientations of the raw shape memory alloy are arranged whenthe directions of reversible slip motions of the respective crystalgrains are arranged in the expected operational direction. Hereupon, theorientation of crystal grain means the one where a reversible slipdeformation due to the martensitic transformation is easy to occurpractically such as one of orientations of variants and the like, butnot necessarily one and the same orientation from the view point of thecrystallography.

The step of introducing a slide deformation to the crystal grains andthat of arranging the directions of reversible slip motions of the scrystal grains may be repeated a required number of times. Generally itsuffices to repeat one to three times.

In the method of treating a shape memory alloy in accordance with thepresent invention, it is preferable to take place a step of running-in,after having rearranged the crystal grains of the raw shape memory alloyalong the direction which is suited for the reversible deformation ofthe alloy in the expected operational direction as stated above, inorder to obviate instability of the alloy which appears in the initialstage of its repetition movement. This running-in step is a processwhich aims for the same effect as the training process which has beenemployed in the conventional shape memory treatment.

Preferably, the running-in step is performed, after arranging thedirections of reversible slip motions of the respective crystal grainsof the raw shape memory alloy in the expected operational direction, bysubjecting the raw shape memory alloy to a heat cycle between atemperature of M_(f) point or below and a temperature at which only ahigh level of plastic deformation is relaxed, while controlling a stressapplied to the raw shape memory alloy without restraining the strainintroduced in the raw shape memory alloy. In general, it is preferablethat a few to several tens cycles of the heat cycle is applied to theraw shape memory alloy. In accordance with the running-step, a workhardening and a structural defect having an elastic energy field whichcontribute to the dimensional stability and two-way shape memory effectof the alloy can be stored in the microstructure at and around thecrystal grain boundaries to the desired degree and thereby theinstability of the alloy which appears in the initial stage of itsrepetition movement can be dissolved.

It has not been yet fully elucidated theoretically what phenomenonoccurs in the shape memory alloy and why the alloy exhibits variousexcellent properties as stated above when the treatment in accordancewith the present invention is carried out. However, to make the presentinvention easily understood, a supplementary explanation will be givenhereunder on the basis of a hypothesis the present inventor holds atpresent.

It is considered that in a polycrystalline shape memory alloy eachcrystal performs as a single crystal, while the structure at and aroundthe crystal grain boundaries connects the crystals with each other.Therefore, in case orientations and sizes of the crystals are random,when the respective crystals present large deformations due to thesuperelasticity and shape memory effect, the structure at and around thecrystal grain boundaries is subject to structural contradictions causedby the deformations of the crystals. The conventional shape memoryalloy, treated with an ordinary shape memory treatment aftermanufactured by ordinary working such as casting, hot working and thelike, is polycrystalline and random in the crystal orientations andsizes thereof, and some of the crystals thereof have been destroyed bystrong working. Such circumstances constitute obstacles disturbing asmooth deformation and shape recovery of the alloy, and thereby aconsiderable force is required to deform the alloy, even when at atemperature sufficiently low for the martensitic transformation to becompleted. Therefore, satisfactory shape memory effect can not beachieved when it is used as an actuator, even after the shape memorytreatment.

The shape recovery force within the crystal grain is strong and hasenough magnitude to deform plastically and destroy the structure at andaround the crystal grain boundaries which constitutes a connectionbetween crystal grains and the crystal grains which is not yet in theshape recovery state. This may explains the reason why the conventionalshape memory alloy soon loses the memory of the imparted shape andbecomes hard, with the operational strain thereof decreasing, when it issubject to repetitions of a large deformation and shape recovery. It maybe because the interior of the shape memory alloy is changed little bylittle due to the great shape recovery force. Especially, in the casewhere the shape memory alloy performs the shape recovery when it issubject to a large deformation and restrained in the deformedconfiguration, the shape recovery forces of the respective crystalgrains act on the interior of the alloy material at a stretch and theshape memory alloy deteriorates rapidly. The fact is that, in case ofthe conventional shape memory alloy, the superelastic spring and thelike, the above-mentioned defect should be covered up by practicingstrong working to cause work hardening in the alloy, and consequentlyconstructing the internal structure in the alloy where the huge shaperecovery forces of the crystals are restrained.

On the other hand, in accordance with the present invention, the sizesof the crystal grains being made even and the orientations thereof beingarranged along the predetermined direction, even if a huge shaperecovery force is produced in each crystal grain, there is no part inthe alloy where an excessive deformation is produced and the internalstructure of the alloy becomes hard to break. Besides, if the respectivecrystal grains are adequately fine, structural contradictions produceddue to the differences between the orientations of the respectivecrystal grains or the like are small, and the crystal themselves becomeshard to break. Moreover, in such a fine-grained material, since thevolume proportion of the structure at and around the crystal grainboundaries to that within the grains is comparatively larger, theability to absorb the structural contradictions is high. Further,probably as the structure at and around the crystal grain boundariesexhibits properties like those of an amorphous material, it can beconverted into a shape memory alloy in the shape of a wire or sheet,etc. which is sufficiently ductile over a wide strain range, even in thecase where it is brittle as a raw material. Though the respectivecrystal grains are fine, since the crystal orientations are arrangedalong the specific direction, a comparatively large shape memory effectcan be extracted from the shape memory alloy. The force required todeform the shape alloy is small, since the orientations of therespective crystals along which they are easy to move are arranged alongthe specific direction. Because the volume proportion of the structureat and around the crystal grain boundaries to that within the grains iscomparatively larger, large elastic energy can be stored at and aroundthe crystal grain boundaries without employing the measures ofdepositing impurities there, or the like, and thereby a stable and largetwo-way shape memory effect can be obtained as well as the property thata force required to deform the alloy is small.

When crystal orientations of a shape memory alloy are random, the largerthe average grain size of the shape memory alloy is, more conspicuouslythe shape memory effect occurs. However, in that case, stability as amaterial is deteriorated. The reason for it is thought that structuralcontradictions are liable to be produced in the alloy due to the largegrain sizes and random crystal orientations, causing changes ofstructure in the alloy. For instance, a treatment for a shape memoryalloy generally called high temperature treatment has been known whereinthe shape memory alloy is annealed sufficiently at a high temperature.According to this treatment, because the crystal grain sizes becomelarger, a large shape memory effect can be induced, but loss of thememorized shape, generation of a permanent deformation and decrease ofthe operational strain, etc. are caused soon with a deformation-recoverycycle number. Accordingly, though large operational strains can beextracted, the alloy becomes functionally unstable, and thereby nowadaysthis high temperature treatment is not put to practical use. On thecontrary, when the crystal grains are fine, though the magnitude of theshape memory effect decreases relatively, the shape memory alloy becomesmaterially stable, since structural contradictions produced in the alloydue to the movement of the respective crystals become small and affectless the respective crystals.

Besides, as stated before, with a fine-grained structure, the volumeproportion of the structure at and around the crystal grain boundariesto that within the grains is larger, as compared with in the case of acoarse-grained structure. Accordingly, the properties of the boundariesof crystal grains appears outside conspicuously. It is considered thatthe structure at around the crystal grain boundaries is in disorder andamorphous like properties are dominant there, as compared with theinterior of the crystal grain which has a well-ordered atomicarrangement. The metal structure at and around the crystal grainboundaries and that within the grains are structurally differentmaterial, though they make little difference in composition. Naturally,the properties of the metal structure at around the crystal grainboundaries must differs very markedly from those of the metal structurewithin the grains. While it is easy to impart a deformation related tothe shape memory effect to the structure within the crystal grains, itis difficult to impart such deformation to the structure at around thecrystal grain boundaries, since it is constrained, getting between thecrystal grains, and has poor reversible deformability. Therefore, it isconsidered that the metal structure at and around the crystal grainboundaries and that within the grains are two different materials. As amatter of course, transformation points within crystal grains differfrom those at and around the crystal grain boundaries. It is thoughtthat the process of rearranging the crystal orientations along thespecific direction in the present invention uses the aforesaidproperties of the crystal grain boundaries and therearound.

Most of conventional shape memory alloy production methods and shapememory treatments control strains of the shape memory alloy to define ashape of a finished shape memory alloy and a memorized shape. On thecontrary, one of the distinguishing characteristics of the presentinvention is that most of the main processes thereof are carried out ina state where not the strain but the stress is controlled, allowing theraw shape memory alloy to deform freely. By not controlling the strain,the present invention utilizes the property of the shape memory alloythat the alloy itself reconstructs the internal structure thereof to beadapted for the movement circumstances thereof.

Besides, since the entire treatment process is carried out in rapiddynamic heating and cooling operations, long spells of heat treatment isnot required unlike in the case of conventional treatments, though theprocedure is comparatively complicated. Therefore, a high speed andconsecutive large-scale process for treating a shape memory alloymaterial can be attained which provides a high-performance shape memoryalloy.

Shape memory alloys, more particularly Ti—Ni based and Ti—Ni—Cu basedshape memory alloys are not ordinary alloys consisting of two or moremetals simply mixed together but intermetallic compounds having strongcovalent bonding character. Due to the strong covalent bondingcharacter, they have characteristics like those of inorganic compoundssuch as ceramic and the like, though being metal. Free electrons arerestrained considerably within them because of the strong covalentbonding as compared with the case with metallic bond. Smallness of thefree electron movement within them is supported by their properties ofpoor heat conduction and high electric resistance, though they aremetal. The difficulty of free electron movement makes it hard for thefusion and reorganization of the electron cloud to occur. This is astrong reason that Ti—Ni and Ti—Ni—Cu based shape memory alloys arebrittle materials which are hard to plastically deform. Though thetreatment in accordance with the present invention can be applied to allkinds of shape memory alloys, particularly it is very effective whenapplied to shape memory alloys, such as Ti—Ni or Ti—Ni—Cu based shapememory alloys or the like, which have strong covalent bonding characterand are brittle as raw materials. When the treatment is applied to suchmaterials, the service life, the moving range and the dimensionalstability thereof are remarkably improved especially in repetitionaction under a heavy load, and the ductility thereof is also improved.Moreover, it becomes possible to use alloy compositions which hithertohave been considered to be no use for shape memory alloys, as alloyswith them being hard to work or being too brittle even though possibleto be worked. Accordingly, it can be expected to create new shape memoryalloys which have unprecedented properties.

In another particular aspect of the method of treating a shape memoryalloy in accordance with the present invention comprises the steps ofsubjecting a raw shape memory alloy having an anisotropy in an expectedoperational direction to a high level of deformation by means of astress in the expected operational direction at a very low temperatureat which the austenite phase does not remain in the raw shape memoryalloy so that a slide deformation is introduced into the crystal grainsof the raw shape memory alloy which have been transformed completelyinto the martensite phase, within a reversible range along the directionof the stress; heating the raw shape memory alloy to a temperaturebetween A_(f) and the recrystallization temperature with a stressapplied to said raw shape memory alloy in the expected operationaldirection so that the directions of reversible slip motions of therespective crystal grains of the raw shape memory alloy are arranged inthe expected operational direction.

In this case, the raw shape memory alloy is not necessarily should havesubstantially uniformly fine-grained crystal structure. According tothis aspect, also the crystal orientations are arranged along thedirection suitable for the expected operational direction withoutbreaking the structure of the shape memory alloy, as in the aforesaidaspect.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and the other objects, features and advantages of thepresent invention will become apparent from the following detaileddescription when taken in connection with the accompanying drawings. Itis to be understood that the drawings are designated for the purpose ofillustration only and are not intended as defining the limits of theinvention.

FIG. 1 is a schematic presentation of transformation points andtemperature singular points of a raw shape memory alloy in a firstembodiment of the treatment in accordance with the present invention.

FIG. 2 is a presentation of the transformation points and thetemperature singular point S, etc. of a Ti—Ni—Cu based shape memoryalloy appearing upon heating which are actually measured with a DSC(Differential scanning calorimeter).

FIG. 3 is a presentation of the cryogenic temperature singular point Bof a Ti—Ni—Cu based shape memory alloy actually measured with a DSC.

FIG. 4 is a cross-sectional view showing step 1 of the first embodiment.

FIG. 5 is a cross-sectional view showing step 2 of the first embodiment.

FIG. 6 is a cross-sectional view showing step 3 of the first embodiment.

FIG. 7 is an example of stress-strain diagram in the step 3 of the firstembodiment.

FIG. 8 is a cross-sectional view showing step 4 of the first embodiment.

FIG. 9 is a presentation of the comparison of the stress-strain curve ofthe shape memory alloy obtained by the first embodiment with those ofconventional shape memory alloys.

FIG. 10 is a explanatory drawing showing the test condition formeasuring the characteristics of FIG. 9.

FIG. 11 is a perspective view showing a state where a raw shape memoryalloy is subject to a twisting deformation in step 2 of a secondembodiment of the treatment in accordance with the present invention.

FIG. 12 is a cross-sectional view showing a state where the raw shapememory alloy torsionally deformed in the step 2 of the second embodimentis heated under restraint.

FIG. 13 is a perspective view showing step 3 of the second embodiment.

FIG. 14 is a perspective view showing step 4 of the second embodiment.

FIG. 15 is a perspective view showing step 5 of the second embodiment.

FIG. 16 is a perspective view showing step 6 of the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will hereunder be described in conjunction withpreferred embodiments of the invention which are shown in the drawings.In the drawings like reference numerals are used throughout the variousviews to designate like parts.

FIGS. 4 through 9 show a first embodiment of the method of treating ashape memory alloy in accordance with the present invention. In thisembodiment, it is expected that upon using the finished shape memoryalloy, the alloy is contracted to a memorized length, namely originallength upon heating, while it relaxes upon cooling, expanding to aoriginal deformed length, that is, a length with an elongationdeformation from the memorized length. Therefore, the expectedoperational direction is a tensile direction in this embodiment. In thisembodiment, a Ti—Ni based shape memory alloy material and a Ti—Ni—Cubased shape memory alloy material containing 8 to 12 atomic percent Cuare used as raw shape memory alloys 1.

The treatment in this embodiment basically consists of three stages. Thefirst stage (Steps 1 and 2) is a process of producing fine-grainedanisotropic crystals. The second stage (Steps 3 to 5) is a process ofrearranging the respective crystals to conform to the expectedoperational direction of the alloy. The third stage (Step 6) is arunning-in process of dissolving instability of the alloy which appearsin the beginning of the reiterative operation. However, the essence ofthe treatment resides in the first and second stages. Upon completion ofthe second stage, a high performance shape memory alloy for actuators isalready obtained. Hereunder the treatment of this embodiment will beexplained in order.

(Preparatory operation)

Raw shape memory alloy materials manufactured by casting and hot workingare annealed, and thereafter worked into a desired size by drawing witha die or cold rolling. From the worked shape memory alloys raw materialspecimens H which are left as work-hardned and canonical specimens Nwhich are annealed sufficiently at about 900° C. in accordance with JIS(Japanese Industrial Standard) are prepared. The specimens H and N aresubject to a consecutive and slow heat cycle, and changes of theirspecific heat, electrical resistance, size, hardness, structure and thelike are observed, respectively, and the transformation points andsingular points of the raw shape memory alloys are measured. FIG. 1schematically shows the general relationships between the transformationpoints and the singular points of the raw shape memory alloys. Thenumeric values in the figure represent only a rough standard. Thetemperatures of the transformation points and singular points varyconsiderably according to kinds of raw shape memory alloys. FIGS. 2 and3 shows examples of actual measurement data of DSC.

With regard to the temperature range of heat cycle used for themeasurement, the maximum heating temperature is selected to be about800° C. and the minimum cooling temperature is selected to be −196° C.which is the temperature of liquid nitrogen. From the specimen H aswork-hardened, mainly the temperature singular point S and therecrystallization temperature R are observed. Here, the temperaturesingular point S is an inflection point of physical propertiesrepresenting transformations such as the specific heat, electricalresistance, hardness and the like which is observed between thetemperature range D where a high level of plastic deformations arerelaxed and the recrystallization temperature R (the temperature range Dwill be discussed later in more detail). At present, the inventorconsiders that this temperature singular point S is associated with thetransformation of crystal grain boundaries. From the specimen N withwhich the recrystallization is performed by heating, the temperaturesingular point B is observed as well as the transformation points A_(s),A_(f), M_(s) and M_(f) which are associated with the shape memoryeffect. The temperature range D where only a high level of plasticdeformation is relaxed is observed as the difference in the specificheat between the specimens N and H. The temperature singular point B isan inflection point of physical properties representing transformationssuch as the specific heat, electrical resistance and the like which isobserved in the sub-zero temperature range and considered as atransformation point in the sub-zero temperature range. Though also inthe specimen H, sometimes such singular point is observed, it is notdistinct as in the case with the specimen N and its temperature isliable to differs a little from that observed in the specimen H, perhapsdue to the internal stress. Therefore, as the transformation points,those observed in the specimen N are employed, except the temperaturesingular point S, the temperature range D and the recrystallizationtemperature R.

Though the temperature singular point B varies with the composition ofalloys, in most cases it exists in a very low temperature range of −40°C. to −150° C. which is difficult to obtain without liquid nitrogen orthe like. Accordingly, it is difficult to find the temperature singularpoint B under an ordinary metallurgical measurement environment. In someconditions of materials the temperature singular point B cannot beconfirmed clearly. Accordingly, there is very little literature whichrefers to it. However, this temperature singular point B is anparticularly important temperature in this embodiment. It seems that theM_(f) point measured with the DSC, etc. is principally that of theinterior of the grains which occupy the great portion of the crystals ofthe raw alloy. However, since the crystal grain boundaries arerestrained between crystals having different orientations, it isconsidered that even on the M_(f) temperature or below there stillexists a component which remains as in a state near austenite phase,namely the retained austenite phase. Besides, since the elastic energylevel at the crystal grain boundaries can be high because of workhardening due to plastic deformations and depositions of impuritieswhich are peculiar to the crystal grain boundaries, it is no wonder thatthe M_(f) point of the structure at and around grain boundaries alonelies at a lower temperature. The present inventor thinks that thetemperature singular point B which is much lower than the M_(f) pointmeasured with the DSC is a M_(f)-point-like transformation point of thestructure at and around the crystal grain boundaries. According to thedata measured with the DSC, in most cases the respective transformationpoints and temperature singular points appear as gently-slopinginflection points and it is rarely the case that they have a distinctpeak. The reason for this is thought that the raw alloys measured arepolycrystalline substances each having crystals which are divergent intheir sizes, orientations and conditions under which they areconstrained. As a matter of fact, the temperatures which are commonlycalled transformation points are also represented by the central oraverage values of transformation temperature ranges having a certainwidth, respectively.

(Step 1)

A raw shape memory alloy material 1 manufactured by casting and hotworking is annealed, and thereafter is subject to a high level ofdeformation so as to be formed into a wire shape by cold working, insuch a manner that a great deformation extends sufficiently to theinterior thereof and an anisotropy in the tensile direction remainstherein. To be concrete, as shown in FIG. 4, the raw shape memory alloy1 is subject to wire drawing with a die 2, repeatedly to the limit ofwork hardening at ordinary temperature or a cryogenic temperature withliquid nitrogen. By use of the die 2, external force are applied to theraw shape memory alloy 1 from every direction, and thereby most of thealloy crystals which have been produced upon the solidification of theingot of the alloy or subsequent hot working and which are random insizes and orientations are broken. However, even the raw shape memoryalloy 1 is worked as such, since there is a degree of freedom in thetensile direction, a martensite-like component which causes contractionremains in the structure of the alloy. This component has an anisotropyin the tensile direction and becomes an important element which providesthe crystals with a growth direction upon recrystallization in the step2 which is explained hereunder. It is considered that such state of theraw shape memory alloy 1 after the cold working is amorphous-like onewhere the crystals are crushed almost completely, with the anisotropybeing left in the longitudinal direction.

Though the cold working may be performed at ordinary temperature asstated before, it is preferable that it is performed at a cryogenictemperature, such as that of liquid nitrogen, which is sufficientlylower than the temperature singular point B. The purpose is to transformnon-martensite structures remaining in the alloy, even if the amount ofthem are very small, to the martensite completely. In general, the socalled martensite finished point M_(f) is the temperature which ismeasured with respect to a specimen completely annealed, and in actualworked shape memory materials there remain a considerable amount ofnon-martensite structures even at that temperature. The non-martensitestructures may be retained austenite, structure resulted from workhardening or the like. In this step 1 it is essential that the raw shapememory alloy 1 is worked so that the non-martensite structures remain aslittle as possible. If the austenite or the like component remains, incertain conditions of the worked alloy, sometimes it makes it possiblefor reversible slips to occur in the alloy, even if they are partial,and disturbs recrystallization with an anisotropy, and consequentlymaking the following processes incomplete. This may eventually exerts abad influence on the service life of the shape memory alloy with regardto the shape recovery rate and elongation thereof. Care should be alsotaken to a temperature rise due to work heat of the die 2. Particularlyin case of Ti—Ni and Ti—Ni—Cu based shape memory alloys, the deformationresistance has a tendency to largely depend on the strain rate andthereby heat generation is easy to occur. With great stresses and atemperature rise, since the martensite and the austenite are present ina mixture, the martensite which is weaker in strength than the austeniteis broken with priority and the austenite is liable to remain. It isdifficult for the austenite which has completely transformed to have adirectionality, and thereby an anisotropy in the tensile directioncannot be obtained. Therefore, care should be taken to the high speedwork. Severe cold working at a temperature which is sufficiently lowerthan the temperature singular point B, such as that of liquid nitrogen,can realize the state which is almost ideal in this step. Under such atemperature, since almost the entire of the austenite in the raw shapememory alloy 1 is transformed to the martensite, the entire of thestructure of the raw alloy 1 is broken uniformly except the martensitehaving the orientation suitable for the tensile direction. Stressesexerted by the remaining martensite become a factor presiding over theanisotropy of the recrystallization in the step 2 which will beexplained hereunder.

By the way, besides wire drawing, cold rolling and shot blasting areeffectual as the severe working. If the raw shape memory alloy ismanufactured by sputtering or plating, it is thought that the structurethereof is already in an amorphous-like state, and thereby it is notnecessary to break the crystal structure thereof by the severe coldworking as in the step 1.

(Step 2)

The raw shape memory alloy 1 which has undergone the step 1 is fixed toa restraining device 3 at the both ends thereof, as shown in FIG. 5,with appropriate tension applied thereto. Consequently, the raw shapememory alloy 1 is subject to a stress in the tensile direction with thestrain thereof restrained. Under such condition the raw shape memoryalloy 1 is heated for a few seconds to several minutes to thetemperature at which the recrystallization begins or a little above. Bythis, a substantially uniformly fine-grained equiaxed crystal structurewith an anisotropy in the tensile direction is produced. The reason isthat, it is thought, a large internal tensile stress is caused byheating due to the anisotropy in the tensile direction, and therecrystallization advances preferentially in such a direction that theinternal stress is gradually relieved. When the raw alloy 1 is processedinto such a state, the final size stability and movement property of theshape memory alloy is improved. There is not a severe restriction as tothe magnitude of the stress applied to the raw shape memory alloy 1prior to heating and restraining, because similar effects can beexpected in a wide range thereof A deformed component which can berestored upon heating remains to some extent in the raw shape memoryalloy 1 which has been subject to the severe cold working as in the step1. Therefore, in this step, even if the alloy 1 is not subject to astress and just restrained in its length so as not to become loose inthe absence of a load, it attempts to contract upon heating, thereby astress being produced therein, and consequently almost the same resultcan be attained as when the alloy is subject to a stress and the strainproduced therein is restrained as stated above. Accordingly, such acondition can be employed as well. On the other hand, when the raw shapememory alloy 1 is restrained with a high level of stress appliedthereto, the excessive stress is relaxed during the recrystallizationand thereby it has little effect, but the finished shape memory alloy isdeteriorated in the size accuracy. For instance, when the alloy in theshape of a wire is subject to an excessive tensile stress, it becomesthin. Basically, it is enough if the raw shape memory alloy 1 is loadedwith an adequate stress in the tensile direction when the recoveryrecrystallization begins. What is essential is that the raw alloy 1 issubject to as little stress or constrain as possible except those in thetensile direction during the recrystallization. Actually in thisembodiment, the raw alloy 1 is constrained with a stress of 10 to 100Mpa applied thereto.

By the way, when mass production of the shape memory alloy in accordancewith the present invention is considered, using a tunnel kiln, a similarprocess can be achieved, performing a similar heating treatment, whilethe raw shape memory alloy is subject to a stress by keeping an externalforce acting thereon instead of restraining it as stated above. However,in that case, perhaps because the obtained crystal structure which isfine-grained with the crystal orientations arranged is partly destroyed,a finished shape memory alloy is not so excellent in it properties as inthe case where the raw alloy is restrained, and the control of thestress is difficult.

It is thought that the effect of the restraint with the stress appliedto the raw alloy is as follows. In the material which has undergone thestep 1 the formation of crystals due to the recrystallization is causedwith priority in a part where a greater deformation is imparted suchthat the lattice structure is more disturbed and the stress fieldbecomes stronger. When the crystal formation is achieved with the stressdue to the external force in the tensile direction applied to the alloy,both the interior of the crystal grains and the grain boundaries come toa state where residual stresses and strains are eliminated inequilibrium with the stress. When from the raw alloy 1 thus processedthe stress is removed by removal of the external force or constraintafter cooling, the equilibrium of the internal stress which has beenrelaxed is disturbed and the raw alloy 1 becomes a material whichstructurally has a residual stress field being directional in thetensile direction therein. Besides, it is thought that generally when acrystal is formed, the impurity concentration is far richer outside thecrystal being formed than the inside thereof and at last the impuritiesconcentrate at the grain boundary (constitutional supercoolingphenomenon). The impurities may be substances such as carbon, carbide,oxide and the like which differ in composition from the most part of theraw alloy 1. By means of the step 2, the impurities settle at positionswhere they are stable under the stress, and after cooling, with thestress removed, they are located partially in the tensile direction. Itis thought that such anisotropy of the recrystallization and partialityin the tensile direction due to the impurities constitute an elasticenergy barrier which prevents a plastic deformation from occurring and acause of a stress field which induce the two-way shape memory effect.Moreover, the anisotropy facilitates the next step 3 and subsequentsteps. As a matter of fact easiness of the two-way shape memory effectappearance depends on the carbon concentration.

In the step 2, the stronger covalent bonding property of the alloy is,the easier it is to produce fine crystal grains therein, perhaps becausethe less the thermal conductivity of the alloy is. At present it iseasier to produce fine crystal grains in Ti—Ni—Cu based alloys than inTi—Ni based ones. Though it is strictly a matter of comparison, when theheating temperature is too high or the heating time is too long, thefinished shape memory alloy is inferior in properties as an actuator andunstable as a material, perhaps because the structure at around grainboundaries are lost or the crystal grains become too large. In general,there is a tendency that the larger the crystal grain sizes of shapememory alloys are, the larger the shape recovery strain and the shaperecovery force are. However, in this treatment method, a good result isobtained when the raw alloy crystal structure is made as uniformlyfine-grained and equiaxial as possible, having the grain size of aseveral microns or less, which grain size is small for ordinary metalmaterials. The reason for this is that the subsequent process ofarranging crystal orientation is thought to be more important and thecrystal grains are easy to rotate when their sizes are small andsubstantially uniform. Besides, it is thought that there is a grain sizewhich is suitable for the repetition movement of the shape memory alloyand stable and it seems to be comparatively small. The optimum grainsize for the treatment in accordance with the present invention alsodepends on material, shape and size of the raw alloy.

(Step 3)

After the completion of the step 2, as shown in FIG. 6, the raw alloy 1is newly subject to a large tensile force F₁ under a free tensilecondition without constraint with regard to the cross-sectionaldirection at a cryogenic temperature which is sufficiently lower thanthe temperature singular point B and at which it is completely inmartensite state, until the reaction force increases rapidly, and adeformation is imparted thereto in the tensile direction. Sincesometimes the temperature singular point B is changed by a great stressand deformation, the above described cryogenic temperature is obtainedusing dry ice or liquid nitrogen. As such, it is thought that both theinterior of the crystal grains and the grain boundaries are completelyin the martensite state. The principal point is that the raw alloy 1 isdeformed in a state where neither within the crystal grains nor thegrain boundaries the austenite phase remain. Especially the interior ofthe crystal grain, being very soft, is readily deformed by the externalforce and does not resist it in the range where the reversible slip ofthe atoms occur as described before. This huge deformation strain withinthe crystal grain reaches to tens to hundreds times the elastic strainseen with common metals. On the other hand, the structure at and aroundthe grain boundaries which is situated between crystal grains havingdifferent orientations and is restrained by them cannot move freely,unlike the structure within the crystal grains, and consequently, withdeformations of the neighboring crystal grains, is deformed particularlyin a direction wherein the crystal grains slide against each other inaccordance with the external force. This huge slip deformation is, forthe structure at and around grain boundaries, a plastic deformationwhich exceeds the reversible slip range. In the alloy 1 as a whole theexternal force is relieved and a deformation is produced in such a waythat the strain are stored at the structure at and around the grainboundaries. During this process it is necessary not to apply the forceto the raw alloy 1 so excessively that the plastic deformation reachesto the interior of crystal grains. The limit of the force is easilylearned by observing consecutively a stress-strain diagram as shown inFIG. 7. In the case that the raw shape memory alloy 1 is in the shape ofa wire as in this embodiment, when it undergoes a free tensiledeformation without external forces other than that in the tensiledirection applied thereto at a cryogenic temperature, the deformationoccurs with a comparatively small force to a certain point, but thenabruptly the reaction force increases, and so the stress. The limit ofthe force is learned from the point at which the stress increasesabruptly. In the event that an excessive deformation is imparted to theraw alloy 1 in disregard of the magnitude of the reaction force, theplastic deformation reaches to the interior of the crystal grains,causing a fear of internal defects occurring in the alloy and its abruptrupture. In general, it is preferable to apply a stress of 300 to 500Mpa to the raw alloy 1.

In order to obtain more excellent properties in the tensile direction,preferably a free tensile deformation wherein there is no restraintexcept in the specific direction, or the like, is subject to the rawalloy 1, as in this embodiment. When an alloy having comparatively smallcross section is deformed in such state, rotations and slips between thecrystal grains occur easily, because constraint is small in the crosssection. On the contrary a high level of deformation, such as that bywire drawing, which restrains even movement of the crystals in the rawalloy decreases the effect of this step.

(Step 4)

After the completion of the step 3, the raw shape memory alloy 1 isheated to the vicinity of the temperature singular point S at a heatingrate which does not cause the deposition and diffusion (for instance,100 to 200° C./min) with a tensile fore F₂ which is smaller than that inthe step 3 being applied thereto, as shown in FIG. 8, in a free tensionmanner without restraint in the cross-sectional direction thereof, andthereafter cooled. The force F₂ is selected to be such a small one thatit will not cause a deformation continuously in the tensile direction.In this step, also, it may be better to say that the strain is notimparted forcibly but the stress is controlled. In general, preferablythe stress is 100 to 200 Mpa. Similar result is obtained when the rawalloy 1 is heated to the temperature singular point S under constraintwith being pre-deformed in the tensile direction, since a shape recoveryforce is produced. But in this case the strain under constraint isdifficult to control. In this step the interior of the crystal grainsbecome the austenite phase which is hard, and thereby the structure atand around grain boundaries is brought into a state where it isrestrained. At the temperature S, the structure within the critalgrains, having no excessive deformation and being comparativelywell-ordered in its atomic arrangement, is stable and seldom makes achange. On the other hand, the structure at and around the grainboundaries, where a high level of crystalline distortions due to thelarge plastic deformation have been induced in the step 3, is thought tobe higher than that within the crystal grains in the elastic energylevel or the level of mechanical energy which tries to restore thecrystals to their original state. Therefore, the structure at and aroundthe grain boundaries is liable to undergoes a change like therecrystallization and revert to a more stable status by less heatenergy. Thus in this step 4 the structure at and around the crystalgrains alone selectively undergoes irreversible slip deformations andconsequently the adjoining crystal grains slide along each other so thatthe tensile force from the outside is relaxed. Taking a broader view ofit, it means that, when the respective crystal grains take place areversible deformation due to the shape memory effect, they rotate sothat they are arranged in their orientations and can move more smoothly.In other words, all of the crystal grains are arranged in a direction inwhich the movement of the shape memory alloy in the expected operationaldirection, namely the tensile direction, is obstructed less. Sincecrystal grains of shape memory alloys have many crystal planes in threedimensions, where reversible deformations referred to as variantsreadily occur (for instance, in case of a Ti—Ni based shape memoryalloy, there are as much as twenty four (24) orientations along whichthe deformations referred to as variants can occur), with acomparatively slight rotation each of the crystal grains can settle inthe direction suitable for the deformation in the tensile direction.Once settled in the stable direction, each of the crystal grains cantake place a reversible deformation to the maximum when the alloy as awhole is subject to a tensile deformation. Accordingly, a force rotatingthem further is hardly produced. In other words the alloy becomes stableas a material. In the event that the step 2 is not carried out well andconsequently the crystal grains are uneven in their size, excessivestresses and deformations are produced in the interior of crystal grainswhich lacks conformity and the alloy becomes materially unstable. Incase the load, temperature and heating time are not adequate, thecrystal grains do not rotate, and moreover, the change reaches even theinterior of the crystal grains, and consequently the properties of thealloy become deteriorated.

The phenomenon which occurs in the steps 3 and 4 which is associatedwith the fine-grained polycrystalline material seems to be that similarto the ultra fine grain super plasticity. A great difference between thephenomenon related to the present invention and the ultra fine grainsuper plasticity which heretofore has been known is that in the presentinvention the process is finished before the stage where a continuousdeformation lasts is reached. However, when the alloy is held for alonger time at a heating temperature higher than the singular point Sand deformed slowly, sometimes a large permanent strain is produced.

(Step 5)

If necessary, the step 3 is carried out again with the raw shape memoryalloy 1 which has undergone the step 4. Generally, there is a tendencythat, when the process of the steps 3 and 4 is carried out once, most ofthe crystal grains are successfully arranged in a direction suitable forthe expected operational direction, and even if the process is repeated,the effect is decreased logarithmically with the number of repetitions.However, the result of the steps 3 and 4 differs with alloys and in somecases the number of repetitions delicately affects properties of thefinished shape memory alloy. Therefore, in some cases, as the steps 3and 4 are repeated alternately, the properties of finished shape memoryalloy are improved gradually. The reason for this is thought to be thatin certain cases the intermetallic compound which forms the alloy has asmaller number of orientations in which variants are easily produced,depending on impurities included therein and the composition andhistories thereof. In practice it is preferable to determine the numberof repetitions from results of a operation test for the shape memoryalloy with which all the processes of the treatment have been completedonce. One standard judgement to determine the appropriate number ofrepetitions is to confirm that the stress when the alloy undergoes adeformation at the cryogenic temperature becomes sufficiently smallerthan that in the first step 3 or zero.

(Step 6)

The raw shape memory alloy 1 is repeatedly heated and cooled between amaximum heating temperature and a minimum cooling temperature with aforce applied thereto. The maximum heating temperature is selected to bein the vicinity of the temperature D, and the minimum coolingtemperature is selected to be the M_(f) point or below, preferably acryogenic temperature similar to that in the step 3. The force isselected to be larger than that which is expected to be applied to theshape memory alloy when it is used as an actuator but not so large as todamage it. Though it depends on circumstances, in general a stress of100 to 300 Mpa is thought to be preferable. In this step the movement ofthe alloy 1 by the heating and cooling cycle should not be restrained.It is more effective to set the magnitude of the force to be larger uponcooling than upon heating. This step work hardens the structure at andaround the grain boundaries adequately to secure the dimensionalstability of the alloy and induces an elastic energy field in the alloyin a direction opposite to that of the shape recovery of the alloy dueto the shape memory effect, as is the case with conventional trainingprocesses of shape memory alloys. The completion of this step finishesall the processes of the treatment.

The curve I in FIG. 9 shows an example of a temperature-straincharacteristic of a Ti—Ni—Cu based shape memory alloy obtained by thisembodiment. In FIG. 9 characteristics of conventional shape memoryalloys for actuators (curves II and III) are also shown for comparison.FIG. 10 shows test conditions for measuring the characteristic of FIG.9, wherein relations between the temperature and shrinkage displacement(contraction strain ε) of the respective shape memory alloys 1′ in theshape of a wire are measured in a thermostat (constant temperature oven)controlled at the temperature change 10° C./min with a load of 100 Mpato the shape memory alloys 1′. As shown by the curve I in FIG. 9, as forthe shape memory alloy obtained by this embodiment the temperaturehysteresis is almost zero in a comparatively wide range. Both theconventional shape memory alloy shown by the curve II, which is of ahigh temperature type that operates at a comparatively high temperature,and the conventional shape memory alloy shown by the curve III, whichhas been processed with the medium treatment, exhibit large hystereticcharacteristics.

FIGS. 11 through 16 show a second embodiment of the method of treating ashape memory alloy in accordance with the present invention. In thisembodiment it is expected that the finished shape memory alloy takes theshape of a coil or helical spring, and when used as an actuator, itcontracts to the memorized (original) coil length upon heating, while itrelaxes and elongates to the original deformed coil length at a lowtemperature upon cooling (namely it operates as an extension spring), orit elongates to the memorized coil length upon heating, while it relaxesand contracts to the original deformed coil length at a low temperatureupon cooling (namely it operates as a compression spring). In thisembodiment the expected operational direction is a twisting direction.

(Preparatory operation)

An operation similar to the preparatory operation in the firstembodiment is carried out.

(Step 1)

An operation similar to the step 1 in the first embodiment is carriedout to prepare a raw shape memory alloy 1 in the shape of a wire havingpredetermined diameter. Though an anisotropy in the tensile directionremains in the raw shape memory alloy 1, it has substantially no effecton the characteristics of the finished shape memory alloy to be obtainedat the end.

(Step 2)

The raw shape memory alloy 1 which has undergone the step 1 is twistedsufficiently in the expected operational direction as shown in FIG. 11to receive a twisting deformation, and then, restrained, as it is, by aconstraining device 3 as shown in FIG. 12. Though the twistingdeformation may be achieved at ordinary temperature, it is preferablethat it is performed at a cryogenic temperature which is sufficientlylower than the temperature singular point B for the same reason as inthe first embodiment. Thereafter the raw shape memory alloy 1 is heatedfor a short period of time to the temperature at which therecrystallization begins or a little above while constrained as statedabove. Then a great internal shearing stress is produced in the alloy 1due to its anisotropy in the twisting direction, and therecrystallization occurs preferentially in such a direction that theinternal shearing stress is relieved, and consequently a substantiallyuniformly fine-grained crystal structure having an anisotropy in thetwisting direction is produced.

(Step 3)

The raw shape memory alloy 1 which has undergone the step 2 is subjectto an additional twisting deformation in the same direction with a largetwisting force as shown in FIG. 13 at a low or cryogenic temperature atwhich it is completely in martensite state until the reaction forceincreases rapidly. Hereupon the twisting torque imparted to the rawalloy 1 should be controlled so as to prevent the plastic deformationfrom reaching to the interior of crystal grains as in the firstembodiment. The deformation should be restrained as little as possibleexcept in the twisting direction.

(Step 4)

As shown in 14, the raw shape memory alloy 1 which has undergone thestep 3 is wound around a core bar 4 having a round cross-sectional shapeso that the twisting deformation may not be dissolved. The raw alloy 1may be wound while being twisted. In the drawings, a part where one endof the raw alloy 1 is fixed to the round core bar 4 is denoted at 5″.Whether the finished shape memory alloy 1 is to form an extension springor compression springs depends on the winding direction. FIG. 14 showsthe case where the finished shape memory alloy is to form an extensionspring. In the case that the finished shape memory alloy is to form ancompression spring, the raw alloy 1 is wound around the core bar 4 inthe opposite direction. When the finished shape memory alloy is supposedto form an extension spring, if the raw alloy 1 is wound around the corebar 4 while strongly twisted, it forms a coil shape for itself ratherthan being forcibly wound around the core bar 4.

(Step 5)

Next, while being restrained in the state where it is wound around thecore bar 4 and twisted as shown in FIG. 15, the raw alloy 1 is heated tothe temperature singular point S at a heating rate which does not causethe deposition and diffusion (for instance, 100 to 200° C./min) andthereafter cooled. Consequently, the crystals of the raw alloy 1 isreoriented along a direction suitable for the expected operationaldirection, namely the twisting direction, as is the case with the firstembodiment. Since in the step 4 the raw alloy 1 is subject to a bendingdeformation as well as the twisting deformation, a higher level ofdeformation may be imparted to it, as compared with the firstembodiment, inducing work hardening in some parts of it. Therefore,there are cases where it is preferable to determine the heatingtemperature to be little higher and the heating time to be short inorder to remove excessive work hardening.

(Step 6)

The core bar 4 is pulled out from the raw alloy 1, and at a cryogenictemperature the coil of the raw alloy 1 is deformed so as to beelongated as shown in FIG. 16 when it is of a extension type, while itis deformed so as to be compressed when it is of a compression type.Instead of it, mere cooling the raw alloy 1 to a cryogenic temperaturewhile it is still wound around the core bar 4 is also effective to thesome extent, perhaps because a stress remains in the raw alloy 1. Thereare cases where it improves further the performances of the finishedshape memory alloy to stretch properly the coil of the raw shape memoryalloy 1 which has been obtained as stated above and thereafter to repeatthe steps 3 to 6 several times.

(Step 7)

When necessary, the raw shape memory alloy 1 obtained by the step 6 issubject to a heat cycles of more than a few cycles between a low orcryogenic temperature and the temperature D while the raw shape memoryalloy 1 is subject to a force in the expected operational directionwithout constraining the deformation thereof. This step is a running-inor training process which corresponds to the step 6 in the firstembodiment. Upon completion of this step, all processes of the treatmentis completed.

The present invention can be applied to shape memory alloys which aredifferent in their shapes and movements from those in the aboveembodiments. Even if manners of deformation are different, basicprocesses of the treatment are same.

Although preferred embodiments of the present invention have been shownand described herein, it should be apparent that the present disclosureis made by way of example only and that variations thereto are possiblewithin the scope of the disclosure without departing from the subjectmatter coming within the scope of the following claims and a reasonableequivalency thereof.

1. A shape memory alloy being polycrystallane and having a substantiallyuniformly fine-grained crystal structure, crystal orientations thereofbeing arranged substantially along a direction suitable for an expectedoperational direction wherein said shape memory alloy is selected fromthe group consisting of a wire having a solid round cross section, aplate, and a coil, and wherein said shape memory alloy is prepared by aprocess comprising the steps of: (a) providing a raw shape memory alloyhaving a substantially uniformly fine-grained crystal structure; and (b)arranging crystal orientations of said raw shape memory alloysubstantially along a direction suitable for an expected operationaldirection wherein step (a) comprises the step of: (c) heating said rawshape memory alloy in an amorphous state or a state similar thereto tothe temperature at which recrystallization begins or a little above fora short period of time, with a stress applied to said raw shape memoryalloy in said expected operational direction at least in the stage wherea recovery recrystallization begins, to produce a substantially uniformfine-grained crystal structure with an anisotropy in said expectedoperational direction, while relaxing the internal stress generated insaid raw shape memory alloy in said expected operational direction; andstep (b) comprises the steps of: (d) subjecting said raw shape memoryalloy to a high level of deformation by means of a stress in saidexpected operational direction at a very low temperature at which theaustenite phase does not remain in said raw shape memory alloy so that aslide deformation is introduced into the crystal grains of said rawshape memory alloy which have been transformed completely into themartensite phase within a reversible range along the direction of saidstress; (e) heating said raw shape memory alloy to a temperature betweenA_(f) and the recrystallization temperature with a stress applied tosaid raw shape memory alloy in said expected operational direction sothat the directions of reversible slip motions of the respective crystalgrains of said raw shape memory alloy are arranged in a directionsuitable for said expected operational direction.
 2. A shape memoryalloy as set forth in claim 1, wherein the average grain diameter ofcrystals is 10 microns or less.
 3. A shape memory alloy as set forth inclaim 1, wherein prior to step (c), said raw shape memory alloy issubject to a severe cold working so that the crystal structure thereofis destructed and is brought to a state similar to an amorphous state.4. A shape memory alloy as set forth in claim 1, wherein said shapememory alloy is an intermetallic compound.
 5. A shape memory alloy asset forth in claim 4, wherein said shape memory alloy is a Ti—Ni basedalloy.
 6. A shape memory alloy as set forth in claim 4, wherein saidshape memory alloy is a Ti—Ni—Cu based alloy.
 7. A shape memory alloy asset forth in claim 1, wherein said expected operational direction is atensile direction.
 8. A shape memory alloy as set forth in claim 1,wherein said expected operational direction is a torsion direction.
 9. Ashape memory alloy as set forth in claim 1, wherein said shape memoryalloy is in a form of a wire.
 10. A shape memory alloy beingpolycrystalline and having a substantially uniformly fine-grainedcrystal structure, crystal orientations thereof being arrangedsubstantially along a direction suitable for an expected operationaldirection wherein said shape memory alloy is selected from the groupconsisting of a wire having a solid round cross section, a plate, and acoil, and wherein said shape memory alloy is prepared by a processcomprising the steps of: (g) subjecting a raw shape memory alloy havingan anisotropy in an expected operational direction to a high level ofdeformation by means of a stress in said expected operational directionat a very low temperature at which the austenite phase does not remainin said raw shape memory alloy so that a slide deformation is introducedinto the crystal grains of said raw shape memory alloy which have beentransformed completely into the martensite phase within a reversiblerange along the direction of said stress; (h) heating said raw shapememory alloy to a temperature between the austenite transformationterminate temperature A_(f) and the recrystallization temperature with astress applied to said raw shape memory alloy in said expectedoperational direction so that the directions of reversible slip motionsof the respective crystal grains of said raw shape memory alloy arearranged in a direction suitable for said expected operationaldirection.
 11. A shape memory alloy as set forth in claim 10, whereinthe average grain diameter of crystals is 10 microns or less.
 12. Ashape memory alloy as set forth in claim 10, wherein said expectedoperational direction is a tensile direction.
 13. A shape memory alloyas set forth in claim 10, wherein said expected operational direction isa torsion direction.
 14. A shape memory alloy as set forth in claim 10,wherein said shape memory alloy is in a form of a wire.
 15. A shapememory alloy as set forth in claim 10, wherein said shape memory alloyis an intermetallic compound.
 16. A shape memory alloy as set forth inclaim 15, wherein said shape memory alloy is a Ti—Ni based alloy.
 17. Ashape memory alloy as set forth in claim 15, wherein said shape memoryalloy is a Ti—Ni—Cu based alloy.
 18. A method of making a shape memoryalloy that is polycrystalline and has a substantially uniformlyfine-grained crystal structure, crystal orientations thereof beingarranged substantially along a direction suitable for an expectedoperational direction wherein said shape memory alloy is selected fromthe group consisting of a wire having a solid round cross section, aplate, and a coil, and wherein said method comprises the steps of: (a)providing a raw shape memory alloy having a substantially uniformlyfine-grained crystal structure; and (b) arranging crystal orientationsof said raw shape memory alloy substantially along a direction suitablefor an expected operational direction wherein step (a) comprises thestep of: (c) heating said raw shape memory alloy in an amorphous stateor a state similar thereto to the temperature at which recrystallizationbegins or a little above for a short period of time, with a stressapplied to said raw shape memory alloy in said expected operationaldirection at least in the stage where a recovery recrystallizationbegins, to produce a substantially uniform fine-grained crystalstructure with an anisotropy in said expected operational direction,while relaxing the internal stress generated in said raw shape memoryalloy in said expected operational direction; and step (b) comprises thesteps of: (d) subjecting said raw shape memory alloy to a high level ofdeformation by means of a stress in said expected operational directionat a very low temperature at which the austenite phase does not remainin said raw shape memory alloy so that a slide deformation is introducedinto the crystal grains of said raw shape memory alloy which have beentransformed completely into the martensite phase within a reversiblerange along the direction of said stress; (e) heating said raw shapememory alloy to a temperature between A_(f) and the recrystallizationtemperature with a stress applied to said raw shape memory alloy in saidexpected operational direction so that the directions of reversible slipmotions of the respective crystal grains of said raw shape memory alloyare arranged in a direction suitable for said expected operationaldirection.
 19. The method as set forth in claim 18, wherein the averagegrain diameter of crystals is 10 microns or less.
 20. The method as setforth in claim 18, wherein prior to step (c), said raw shape memoryalloy is subject to a severe cold working so that the crystal structurethereof is destructed and is brought to a state similar to an amorphousstate.
 21. The method as set forth in claim 18, wherein said shapememory alloy is an intermetallic compound.
 22. The method as set forthin claim 21, wherein said shape memory alloy is a Ti—Ni based alloy. 23.The method as set forth in claim 21, wherein said shape memory alloy isa Ti—Ni—Cu based alloy.